Stockholders' Equity - Other (Details) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) item $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares
Equity incentive plans
Granted (in shares)	55
Shares available for issuance	2,700
Stock-based compensation
Stock-based compensation expense (in dollars) | $	$ 4,100	$ 4,700	$ 5,600
Number of shares
Granted (in shares)	55
Forfeited or expired (in shares)	(37)
Expired (in shares)	(40)
Outstanding at the end of the period (in shares)	2,898
Weighted-average exercise price
Granted (in dollars per share) | $ / shares	$ 11.35
Forfeited or expired (in dollars per share) | $ / shares	10.39
Expired (in dollars per share) | $ / shares	11.51
Outstanding at the end of the period (in dollars per share) | $ / shares	$ 11.62
Weighted-average remaining contractual term
Outstanding	6 years 6 months
Granted	6 years
Aggregate intrinsic value
Outstanding at the end of the period (in dollars) | $	$ 1,738
Common Class A
Equity incentive plans
Shares authorized for issuance	7,200
Employee and Directors Stock Options
Equity incentive plans
Granted (in shares)		890	215
Vested at the end of the period (in shares)		1,888
Stock-based compensation
Unrecognized compensation cost related to unvested stock options (in dollars) | $	$ 1,900
Weighted-average periods over which unrecognized compensation cost recognized	1 year 4 months 24 days
Estimated forfeitures (as a percent)	1.50%
Valuation assumptions
Dividend yield (as a percent)	0.00%
Number of shares
Outstanding at the beginning of the period (in shares)	2,920	2,043	1,870
Granted (in shares)		890	215
Exercised (in shares)		(13)	(15)
Forfeited or expired (in shares)			(27)
Outstanding at the end of the period (in shares)		2,920	2,043	1,870
Vested at the end of the period (in shares)		1,888
Exercisable at the end of the period (in shares)		1,888
Weighted-average exercise price
Outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 11.64	$ 11.49	$ 11.23
Granted (in dollars per share) | $ / shares		11.97	13.61
Exercised (in dollars per share) | $ / shares		10.60	10.60
Forfeited or expired (in dollars per share) | $ / shares			10.60
Outstanding at the end of the period (in dollars per share) | $ / shares		11.64	$ 11.49	$ 11.23
Vested at the end of the period (in dollars per share) | $ / shares		11.69
Exercisable at the end of the period (in dollars per share) | $ / shares		$ 11.69
Weighted-average remaining contractual term
Outstanding		7 years 7 months 6 days	7 years 7 months 6 days	8 years 4 months 24 days
Granted		6 years 2 months 12 days	6 years 2 months 12 days
Vested at the end of the period		5 years 9 months 18 days
Exercisable at the end of the period		5 years 9 months 18 days
Aggregate intrinsic value
Outstanding at the beginning of the period (in dollars) | $	$ 1,274	$ 6,740	$ 4,721
Outstanding at the end of the period (in dollars) | $		1,274	$ 6,740	$ 4,721
Vested at the end of the period (in dollars) | $		1,322
Exercisable at the end of the period (in dollars) | $		$ 1,322
Weighted-average grant date fair value of options granted (in dollars per share) | $ / shares	$ 3.39	$ 3.71	$ 4.13
Restricted Stock
Equity incentive plans
Granted (in shares)	154	395	99
Stock-based compensation
Unrecognized compensation cost related to unvested restricted stock (in dollars) | $	$ 2,500
Weighted-average periods over which unrecognized compensation cost recognized	1 year 6 months
Number of shares
Outstanding at the beginning of the period (in shares)	561	494	719
Granted (in shares)	154	395	99
Vested (in shares)	(203)	(328)	(324)
Forfeited (in shares)	(8)
Outstanding at the end of the period (in shares)	504	561	494	719
Weighted-average grant date fair value
Outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 10.58	$ 9.79	$ 9.82
Granted (in dollars per share) | $ / shares	11.19	11.82	12.42
Vested (in dollars per share) | $ / shares	11.80	10.88	10.65
Forfeited (in dollars per share) | $ / shares	13.38
Outstanding at the end of the period (in dollars per share) | $ / shares	$ 10.23	$ 10.58	$ 9.79	$ 9.82
Time Based Restricted Stock and Stock Option
Equity incentive plans
Number of equal annual installments for vesting of awards | item	3
Time Based Stock Option | Black Scholes Pricing Model
Valuation assumptions
Risk-free interest rate, minimum (as a percent)		1.60%	1.76%
Risk-free interest rate, maximum (as a percent)	2.18%	2.44%	2.12%
Volatility, minimum (as a percent)		26.40%	25.80%
Volatility, maximum (as a percent)	25.80%	32.40%	29.50%
Weighted-average expected term	6 years	6 years 2 months 12 days
Time Based Stock Option | Black Scholes Pricing Model | Maximum
Valuation assumptions
Weighted-average expected term			6 years 3 months 18 days
Time Based Restricted Stock
Weighted-average grant date fair value
Vesting period	3 years
Event Based Restricted Stock and Stock Option | Common Class A
Equity incentive plans
Closing price per share to be attained for vesting of awards to begin (in dollars per share) | $ / shares	$ 15.00
Event Based Restricted Stock and Stock Option | Common Class A | Minimum
Equity incentive plans
Number of trading days on which the closing price per share should attain the specified price per share for vesting of awards to begin | item	10
Event Based Stock Option
Aggregate intrinsic value
Unvested options	300
Event Based Restricted Stock
Aggregate intrinsic value
Unvested options	200